-Room design needs improvement. Many of the levels are too bare. This can hurt the game as it makes the levels look overall uninteresting.

-Gameplay is too linear. Sometimes that's not exactly a bad thing, but in your story it's simply point A to point B, and then instantly next room. You should see if you can add a bit more content/variety to spice up the overall gameplay experience.

-Something I did like a bit was the final big scene. This may sound a bit harsh, but that scene was the only part of the game I found somewhat original.

Spoiler below!

This section is a bit more with concerning specific details of the CS:

-The key at the start is IMHO a bit too hard to spot. This can probably differ from person to person. This is just what I think.

-This is part of the problems with level design. The chandeliers are in quite awkward positions in the first room. I've never seen chandeliers put like that before 0.0

-You should consider adding doors/holes at the start of each level, preferably the same ones as the one you just entered in the previous map. Adds a sense of continuity.

Still for a first custom story, I can see you're experimenting with several things, and that'll really help you improve. Looking forward to what you can bring in Part 2

EDIT: Feel free to correct me if there's anything you may disagree about my review. I always willing to put it up for discussion if you feel need for it.

Also played it again, and here's a few more things I didn't notice the first time:

Spoiler below!

-Don't know how I didn't acknowledge this, but the "throw" into the room in the second level was a nice touch =)
-There are a few disconnected models in the water level, mainly the door room in room 1 with the plates.

Honestly, this mod was really lacking. The rooms were very large but contained so little detail to them and were barren, which was very uninteresting to walk through. All I saw as far as variety was room, darker room, flooded room, tiny room with massive hole to fall in. I really suggest adding a little variety to your rooms, such as some scenery, adding more rooms, making the layout less linear and more open to exploration...something to grab attention and make them entertaining to go through. Atmosphere needs work. The dead silence combined with the lack of scenery makes all these rooms really bland and doesn't make them fun to go through. Maybe add some music to the maps, build up monster appearance more and focus less on jumpscares. One jumpscare in a map is fine, but having almost every scare be a jumpscare is not fun and you can predict almost all of them. I noticed a lot of your textures and placements were off, what with parts of the wall fazing through doors and big holes in the walls next to doors, so you should work on how you place walls, doors, objects, ect. The entire second area of the mod is useless, as all you do is get shoved through a door, pick up an illegible note and then move on to the next area. Lengthen out your areas and make them take longer to get through, add a puzzle or key items, some more rooms...as mentioned before, make your maps open for exploration, it helps in building atmosphere.

As a first mod, it's a decent starting point, but you do need to work on most of the foundations to making a map, such as texture, design, layout, atmosphere...the like. Just work on those and your maps will improve greatly.